[Qemu-devel] [PATCH] virtio: limit avail bytes lookahead

["Michael S. Tsirkin" <mst@redhat.com>]

Commit 0d8d7690850eb0cf2b2b60933cf47669a6b6f18f introduced
a regression in virtio-net performance because it looks
into the ring aggressively while we really only care
about a single packet worth of buffers.
To fix, add parameters limiting lookahead, and
use in virtqueue_avail_bytes.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reported-by: Edivaldo de Araujo Pereira <edivaldoapereira@yahoo.com.br>

---

Edivaldo could you please confirm this fixes regression?

diff --git a/hw/virtio-serial-bus.c b/hw/virtio-serial-bus.c
index d20bd8b..a761cdc 100644
--- a/hw/virtio-serial-bus.c
+++ b/hw/virtio-serial-bus.c
@@ -297,7 +297,7 @@ size_t virtio_serial_guest_ready(VirtIOSerialPort *port)
     if (use_multiport(port->vser) && !port->guest_connected) {
         return 0;
     }
-    virtqueue_get_avail_bytes(vq, &bytes, NULL);
+    virtqueue_get_avail_bytes(vq, &bytes, NULL, UINT_MAX, 0);
     return bytes;
 }
 
diff --git a/hw/virtio.c b/hw/virtio.c
index ec8b7d8..f40a8c5 100644
--- a/hw/virtio.c
+++ b/hw/virtio.c
@@ -336,7 +336,8 @@ static unsigned virtqueue_next_desc(hwaddr desc_pa,
 }
 
 void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
-                               unsigned int *out_bytes)
+                               unsigned int *out_bytes,
+                               unsigned max_in_bytes, unsigned max_out_bytes)
 {
     unsigned int idx;
     unsigned int total_bufs, in_total, out_total;
@@ -385,6 +386,9 @@ void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
             } else {
                 out_total += vring_desc_len(desc_pa, i);
             }
+            if (in_total >= max_in_bytes && out_total >= max_out_bytes) {
+                goto done;
+            }
         } while ((i = virtqueue_next_desc(desc_pa, i, max)) != max);
 
         if (!indirect)
@@ -392,6 +396,7 @@ void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
         else
             total_bufs++;
     }
+done:
     if (in_bytes) {
         *in_bytes = in_total;
     }
@@ -405,12 +410,8 @@ int virtqueue_avail_bytes(VirtQueue *vq, unsigned int in_bytes,
 {
     unsigned int in_total, out_total;
 
-    virtqueue_get_avail_bytes(vq, &in_total, &out_total);
-    if ((in_bytes && in_bytes < in_total)
-        || (out_bytes && out_bytes < out_total)) {
-        return 1;
-    }
-    return 0;
+    virtqueue_get_avail_bytes(vq, &in_total, &out_total, in_bytes, out_bytes);
+    return in_bytes <= in_total && out_bytes <= out_total;
 }
 
 void virtqueue_map_sg(struct iovec *sg, hwaddr *addr,
diff --git a/hw/virtio.h b/hw/virtio.h
index ac482be..1278328 100644
--- a/hw/virtio.h
+++ b/hw/virtio.h
@@ -150,7 +150,8 @@ int virtqueue_pop(VirtQueue *vq, VirtQueueElement *elem);
 int virtqueue_avail_bytes(VirtQueue *vq, unsigned int in_bytes,
                           unsigned int out_bytes);
 void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
-                               unsigned int *out_bytes);
+                               unsigned int *out_bytes,
+                               unsigned max_in_bytes, unsigned max_out_bytes);
 
 void virtio_notify(VirtIODevice *vdev, VirtQueue *vq);